Cellulose acetate membranes, which can occur in any shape, have been known for a long time. Thus, U.S. Pat. Nos. 3,133,132, 3,133,137, 3,170,867, 3,283,042, 3,310,488, 3,344,214, and 3,364,288 describe processes for manufacturing semipermeable membranes, suitable for reverse osmosis or for ultrafiltration. All of these processes, however, involve flat membranes exclusively. The high water content of these membranes means however that they cannot be stored in a dry state. In addition, the above membranes cannot be stored for long periods of time without changes occurring in their quality.
European Patent 376,069 describes biocompatible dialysis membranes, suitable for hemodialysis, in the form of flat membranes, tubular membranes, or hollow-fiber membranes, manufactured from polysaccharide esters and corresponding carboxylic acids.
Soviet Patent Application 1435583 describes a process for manufacturing semipermeable membranes composed for example of acetyl cellulose, with acetyl cellulose being treated in two stages either with acetone and water or with acetic acid and water, and with the first processing bath having the higher concentration of acetone or acetic acid.
Japanese Patent 89-028123 describes a process for manufacturing cellulose ester hollow fibers in which the hollow fibers are produced by melt-spinning a mixture of a cellulose ester and an alcohol, and then passing it through a solution containing salt. The aqueous solution can contain acetic acid.
In Japanese Patent Application 57042740, porous membranes are produced using a solution, one cellulose derivative, and a solvent for this cellulose derivative added to a coagulation bath containing more than 60 wt. % of an organic solvent that is a nonsolvent for the cellulose derivative. The solvent for the cellulose derivative can be acetic acid, among other substances.
German Patent 28 35 890 describes membranes for reverse osmosis that can consist of cellulose acetate for example. These membranes are produced from a solution containing the cellulose derivative, an organic solvent, and a tetracarboxylic acid. The organic solvent can be acetic acid, for example.
Japanese Patent 52123983 describes a process in which the thickness of a membrane wall is increased. The membrane is reacted with a solution of 45 to 55 wt. % aqueous acetic acid.
Finally, German laid open application 19 08 344 describes a process for manufacturing cellulose ester membranes from acetic acid, acetone, and an amine salt as the pore-forming substance.
The above membranes leave something to be desired, however, as far as their biocompatibility and/or separating properties are concerned.
Membranes suitable for dialysis should be maximally biocompatible. A number of conditions must be met for this to be the case.
The substances that influence the biocompatibility of a membrane include albumin and .beta.2-microglobulin. .beta.2-microglobulin (molecular weight approximately 11,800) is loosely bonded to the surfaces of all cells with nuclei as a part of the main histocompatibility complex. This complex is responsible for the ability of the body's own tissues to tolerate foreign tissue.
.beta.2-microglobulin is broken down exclusively in the kidney; the daily production rate for a healthy individual is approximately 150 mg. Dialysis patients and uremics, however, have much higher .beta.2-microglobulin serum levels than healthy individuals. It is therefore extremely important to remove .beta.2-microglobulin effectively during treatment.
The albumins likewise belong to the serum protein group and constitute the largest group among them. The albumins maintain colloidosmotic pressure and transport low-molecular-weight substances, both the body's own and foreign substances. They also constitute the protein reservoir of the body.
Since the number of albumins is generally reduced in dialysis patients, it is important to keep albumin losses as low as possible during treatment.
Depending on the area of application, a membrane must be able to exhibit good performance parameters, for example screen coefficient at various filtration rates.
Previously, however, membranes that exhibited corresponding screen coefficients for .beta.2-microglobulin and albumin at a high ultrafiltration rate (high-flux range) did not achieve these figures at average ultrafiltration rates (middle-flux range) or at low to very low ultrafiltration rates (low-flux range).
On the other hand, however, a membrane that worked well in the low-flux range for example suffered a sharp decline in its separating effect in the high-flux range.